This application relates generally to disc drive data storage devices and more particularly to an apparatus and method of compensating for track spacing errors.
Disc drives are the most common means of storing electronic information in use today. Typical disc drives have one or more magnetic media discs attached to a spindle; the spindle and discs are rotated at a constant velocity by a spindle motor. An actuator assembly, attached to a bearing shaft assembly next to the discs, radially traverses over the surface of the discs. The actuator assembly has a plurality of actuator arms, each with one or more flexures extending from the end of each actuator arm. A read/write head is attached to the distal end of each flexure. The actuator assembly is rotated about the bearing shaft assembly by a servo positioner. The servo positioner receives signals from a controller, rotates the actuator assembly, and positions the read/write head relative to the disc surface.
Information is transferred to and from the discs by the read/write heads attached to the flexures at the end of the actuator arms. Each head includes an air bearing slider that enables the head to fly on a cushion of air in close proximity to the corresponding surface of the associated disc. Most heads have a write element and a read element. The write element is used to store information to the disc, whereas the read element is used to retrieve information from the disc.
Discs, to facilitate information storage and retrieval, are radially divided into concentric circles known as xe2x80x9cservo tracksxe2x80x9d or xe2x80x9ctracksxe2x80x9d. Tracks are given a track number, among other identifying information, so that the servo positioner can align the read/write head over desired track. Information is stored or retrieved from the disc after the read/write head is aligned over the desired track. The process of switching between different tracks is called xe2x80x9cseekingxe2x80x9d, whereas remaining over a single track while information is stored or retrieved is called xe2x80x9cfollowingxe2x80x9d.
Each track is linearly subdivided into xe2x80x9csegmentsxe2x80x9d or xe2x80x9csectorsxe2x80x9d. The two most common types of sectors are informational data sectors and servo data sectors. In a typical disc drive, the informational data sectors usually contain information generated or stored by the user such as programs files, application files, or database files. There may be ten to a hundred, or even more, informational data sectors dispersed around a single track.
The servo sectors, on the other hand, contain information that is used by the servo positioner to determine the radial, and linear, position of the head relative to the disc surface and relative to a track center. Servo sectors typically consist of a Gray code field, which provides coarse position information to the servo positioner such as the track and cylinder number, and a servo burst field, which provides fine position information to the servo positioner such as the relative position of the head to the track center. Generally speaking, the burst field creates a specific magnitude signal on one side of the track centerline and a different specific magnitude signal on the other side of the track centerline. The read head can be aligned directly over a track centerline by positioning the read head at the xe2x80x9cnullxe2x80x9d position, or the position in which the sum of the burst field magnitudes cancel each other and equal zero.
Servo sectors are usually embedded between adjacent informational data sectors located on a single track. The servo sector provides positional information to the servo positioner so that the read/write head can be properly aligned over the subsequent informational sector. A clock signal mechanism is used to insure that data intended to be stored in a servo sector does not overwrite data in an information sector (and vice versa).
The number of tracks located within a specific area of the disc is called the xe2x80x9ctrack densityxe2x80x9d. The greater the number of tracks per area, the greater the track density. The track density may vary as the disc is radially traversed. Disc manufacturers attempt to increase track density in order to place more information on a constant size disc. Track density may be increased by either decreasing the track width or by decreasing the space between adjacent tracks.
An increase in track density necessitates increased positioning accuracy of the read/write elements in order to prevent data from being read from or written to the wrong track. Manufacturers attempt to fly the read/write head elements directly over the center of the desired track when the read/write operation occurs to insure that the information is being read from and written to the correct track. Hitting the track center target at high track densities requires that the tracks be as close to perfectly circular as possible when written to the disc surface.
Tracks are usually written on the disc during disc drive manufacturing using one of two means: 1) a servowriting machine, or 2) self-propagated servo writing. A servowriting machine is a large piece of external equipment that writes servo tracks on a disc drive. A typical servowriting machine uses a large actuator with laser interferometer position feedback and a pushpin to position the arm of the disk drive. The write element, which is attached to the arm, is aligned to where the desired track is to be written on the disc surface. A track is written on the disc once the write element is correctly aligned. The head/arm positioner then moves the write element a predetermined distance to the next desired track location. The head/arm positioner, therefore, controls both the track placement and track-to-track spacing.
Although accurate, a servowriter has several drawbacks. First, a typical disc may contain more than 60,000 servo tracks. The process of aligning and writing each track on the disc is very time consuming and expensive. Next, although very accurate at lower track densities, the servowriter cannot meet the accuracy requirements dictated by higher track densities. Finally, track spacing and track shape errors, caused by spindle wobble, vibrations, disc slip, and thermal expansion among others, are introduced during the servowriting process.
The second means of writing tracks on a disc is called self-propagating servo writing. Oliver et al first described this method of servo track writing in U.S. Pat. No. 4,414,589. Several other patents have disclosed slight variations in the Oliver patent, but the same basic approach is used. Under the basic method, the drive""s actuator assembly is positioned at one of its travel-range-limit stops. A first reference track is written with the write head element. The first reference track is then read with the read element as the head is radially displaced from the first reference track. When a distance is reached such that the read element senses a predetermined percentage of the first reference track""s amplitude, a second reference track is written. The predetermined percentage is called the xe2x80x9creduction numberxe2x80x9d.
For example, the read element senses 100% of the first reference track""s amplitude when the read element is directly over the first reference track. If the reduction number is 40%, the head is radially displaced from the first reference track until the read element senses only 40% of the first reference track""s amplitude. A second reference pattern is written to the disc once the 40% is sensed by the read element. The head is then displaced in the same direction until the read head senses 40% of the second reference track""s amplitude. A third reference track is then written and the process continues. The process ends when the actuator arm""s second travel-range-limit stop is reached and the entire disc surface is filled with reference tracks. The average track density is then calculated using the number of tracks written and the length of travel of the head.
If the average track density is too high, the disc is erased, the reduction number is lowered so that a larger displacement occurs between tracks, and the process is repeated. If the track density is too low, the disc is erased, the reduction number is increased so that a smaller displacement occurs between tracks, and the process is repeated. If the track density is within the desired range, the reduction number for the desired average track density has been determined, the disc is erased, and servo tracks are written to the disc by alternatively writing servo and reference tracks. The servo tracks are further divided by alternatively writing servo and informational sectors.
A well-known problem with self-propagating servo writing is called xe2x80x9cradial error propagationxe2x80x9d. The servo system, when writing a new track during self-propagating servo writing, obtains position information by monitoring the signal generated in the read head by the previous track""s servo information. The servo system xe2x80x9cfollowsxe2x80x9d the path of the previous track, and therefore, the track being written inherits any imperfections (caused by spindle wobble, disc slip, changing head fly height, and thermal expansion among others) in the track being followed. The imperfections of the followed track may even be amplified within the written track if, for example, the closed loop gain of the servo positioner is larger than unity at certain frequencies.
Ideally, tracks are perfectly circular and spaced at a specific distance from each other. The imperfections in track shape and track spacing result in xe2x80x9ctrack squeezexe2x80x9d. The non-parallelism of two adjacent tracks is referred to as dynamic or AC track squeeze, whereas track spacing imperfections are referred to as static or DC track squeeze. AC track squeeze refers to the situation in which two adjacent tracks have shape imperfections at different locations around their individual circumferences. The two tracks may be too close together at some points and too far apart at other points.
DC track squeeze, on the other hand, refers to the situation in which the average centers of two adjacent tracks are either closer or farther apart than a nominal distance. In other words, the spacing between the two tracks is incorrect even though the two tracks are perfectly circular. The term xe2x80x9ctrack squeezexe2x80x9d is often used to generally refer to the combination of AC and DC track squeeze. Furthermore, the track-to-track variation of track shape is called the xe2x80x9crelative track shape errorxe2x80x9d, whereas the deviation of the track shape from a perfect circle is called xe2x80x9cabsolute track shape errorxe2x80x9d. The prior art methods of machine servo writing and self-propagated servo writing cannot achieve the accuracy needed for higher track densities because of inherent limitations in controlling track squeeze, relative track shape error, and absolute track shape error.
Yarmchuk et al in U.S. Pat. No. 5,659,436 extensively studied radial error propagation. Yarmchuk proposed that indefinite growth of written in errors are avoided by insuring that the propagation gain is less than unity at all frequencies. Yarmchuk proposed that the gain could be maintained at a value less than unity at all frequencies by carefully choosing the open loop transfer function and/or providing an appropriate reference correction table derived from the position error signal during the write revolution of the previous track. However, the method proposed by Yarmchuk fails to discuss the influence of measurement noise and requires complicated calculations for implementation.
The Yarmchuk method contains an additional drawback. Yarmchuk allows relatively large absolute track shape inaccuracy (i.e., the deviation of the track shape from a perfect circle). In effect, the accuracy obtained by the Yarmchuk method is equal to the track following accuracy of the disc drive, typically about 10%.
Zero Acceleration Path (xe2x80x9cZAPxe2x80x9d) correction is another approach created to eliminate radial error propagation. The basic idea of ZAP correction is to add appropriate correction factors to the measured head position at each servo sector. The correction factors cancel all written in track shape errors, thereby improving the shape of the modified track. The correction factors are typically determined during or after the servo track writing process. The correction factors are then written back onto the discs; usually each servo sector has a dedicated field for storing the correction factors.
A prior art method of determining ZAP correction factors is called xe2x80x9cinverse transformationxe2x80x9d. Inverse transformation guarantees that track squeeze is minimized and that the tracks are circular. In other words, the inverse transformation method guarantees that the relative track shape error (the track-to-track variation) remains small, and that the absolute track shape error (the deviation of the tracks from a perfect circle) also remains small as the self-servo track writing propagates. The major disadvantage of using inverse transformation is that several disc revolutions are required to accurately determine the correction factors. Typically more than eight revolutions are necessary to achieve acceptable accuracy in today""s disc drives.
An increase in track density requires an increase in the accuracy of the ZAP correction factors. Doubling the track density, for example, requires doubling the accuracy of the ZAP correction factors. The number of averaging revolutions must be four times higher to double the accuracy of the ZAP correction factors. Therefore, the total servo writing time will be eight times higher if track density is doubled because twice as many tracks (requiring four times the revolutions for accurate ZAP correction factors) are present on the disc. Each revolution increases the time and cost of servowriting.
A second method of determining ZAP correction factors, called xe2x80x9crecursive estimationxe2x80x9d, was introduced to shorten the amount of time required to correct for track spacing errors. Recursive estimation guarantees that AC track squeeze is minimized, but it does not guarantee an improvement in DC track squeeze. In other words, the recursive estimation method guarantees that the relative track shape error (the track-to-track variation) remains small, but recursive estimation does not guarantee that the absolute track shape error (the deviation of the tracks from a perfect circle) remains small as self-servo track writing propagates.
The xe2x80x9crecursive estimationxe2x80x9d ZAP method is combined with the xe2x80x9cinverse transformationxe2x80x9d ZAP method to overcome the limitations of both methods. A combination of recursive estimation and inverse transformation provides small absolute track shape error and small relative track shape error. Furthermore, a combination of recursive estimation and inverse transformation reduces the number of disc revolutions necessary to determine the ZAP correction factors for each track.
However, the prior art method of recursive estimation does not guarantee small AC track spacing errors if the offset between the read and write head is larger than a few servo track. Furthermore, the prior art method of recursive estimation could only be used during self-propagating servo writing; tracks written by a conventional servowriter could not be corrected by the prior art method of recursive estimation.
Accordingly there is a need for an apparatus and a method of eliminating the track shape errors that occur during self-propagated and conventional servo track writing that overcome the limitations of prior art approaches.
Against this backdrop, an embodiment of the present invention offers a time efficient means to determine and eliminate dynamic track squeeze error. The embodiment of the present invention can be used to correct imperfections in tracks written by a conventional servowriter or in tracks written using self-propagating servo writing. The embodiment of the present invention can be used for various types of storage systems such as magnetic and optical disc drives among others, however, a magnetic disc drive has been used to illustrate the present invention.
Accordingly, a preferred embodiment of the present invention relates to a method of determining dynamic track squeeze error within a single disc revolution. After a servo track is written, the read element is positioned half way between two servo tracks (i.e., half way between the AB null position and the CD null position in a quadrature burst pattern). Positioning information for track following is obtained from either the AB or the CD null set as one disc revolution is completed. During the disc revolution, position measurements are also simultaneously collected from both the AB null and the CD null. The difference between the position measurement from the AB null and the position measurement from the CD null is determined. The difference represents the non-parallelism, or dynamic track squeeze, between the two adjacent tracks.
A preferred embodiment further relates to a method of eliminating dynamic track squeeze error within a single disc revolution. After a servo track is written, the read head is positioned half way between two servo tracks (i.e., half way between the AB null position and the CD null position in a quadrature burst pattern). Positioning information for track following is obtained from either the AB or the CD null set as one disc revolution is completed. During the disc revolution, position measurements are also simultaneously collected from both the AB null and the CD null. The difference between the position measurement from the AB null and the position measurement from the CD null is determined. The difference represents the non-parallelism, or dynamic track squeeze, between the two adjacent tracks caused by track shape imperfections. An appropriate zero acceleration path (xe2x80x9cZAPxe2x80x9d) correction factor is determined and written to the servo sector of one of the tracks. The ZAP correction factor is input to the disc drive servo controller to eliminate track shape imperfections and dynamic track squeeze.
Furthermore, a preferred embodiment of the present invention relates to an apparatus for eliminating dynamic track squeeze error within a single disc revolution. The apparatus includes a servo positioner and controller for aligning the read element half way between two adjacent servo tracks (i.e., half way between the AB null position and the CD null position in a quadrature burst pattern). The servo positioner obtains positioning information for track following from either the AB or the CD null set as one disc revolution is completed. During the disc revolution, the controller simultaneously collects position measurements from both the AB null and the CD null sets. The controller determines the difference between the position measurement from the AB null set and the position measurement from the CD null set. The difference represents the non-parallelism, or dynamic track squeeze, between the two adjacent tracks. The controller of the apparatus determines and writes an appropriate ZAP correction factor to the servo sector of one of the tracks to eliminate track shape imperfections and dynamic track squeeze.
The preferred embodiment of the present invention requires only one disc revolution to determine the appropriate ZAP correction factor. Furthermore, the preferred embodiment of the present invention can be used to eliminate track squeeze errors in servo tracks written by a conventional servo-writer or using self-propagated servo writing.
These and various other features as well as additional advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.